Cyclic adsorption processes are generally practiced in batteries of adsorption vessels comprised of two or more adsorbent-filled vessels arranged in parallel and operated out of phase such that at least one vessel is in the adsorption mode while at least one other vessel is in the adsorbent regeneration mode. In each cycle of the process a series of sequential steps, including adsorption, equalization and regeneration, are carried out in each vessel. To enable the various streams to flow to and from the vessels, the feed, product, and exhaust lines must be provided with valves to permit gas flow through these lines at the appropriate time in the adsorption cycle. Furthermore, cross-connecting lines must be provided between the inlet ends of the vessels and between the outlet ends of the vessels to permit flow between the vessels during pressure equalization steps, and each cross connecting line must be equipped with a valve to control the flow of gas through these lines. All in all, each vessel of the system is provided with at least three valves, and each valve is opened and closed at least once during each cycle of the process. PSA cycles are commonly as short as one minute, accordingly each valve may be required to open and close sixty or more times each hour that the system is in operation. Not only is there considerable wear on each valve over the course of an adsorption run, but considerable energy is expended just to open and close the valves of the system during operation of the plant.
Adsorption processes are inherently batch-type processes. Nonadsorbed gas product is produced only during the adsorption step and desorbed gas product is produced only during the adsorbent regeneration step of the process. Because of this, the desired product is produced, at best, for no more than one-half of each cycle. Since it is often desirable or necessary that a continuous flow of product be available, for example when oxygen is provided for medical purposes, improvements to adsorption systems and processes which provide better product flow continuity are continually sought. Recently, efforts have been made to develop adsorption systems that operate somewhat like continuous process systems. Some of the more promising new adsorption plant designs are based on the principle of rotation. In some designs the adsorption units rotate through stationary gas zones, while in other designs the adsorption units are stationary while gas flow is sequenced through the various units of the system.
U.S. Pat. No. 4,925,464, discloses a simple rotary valve assembly for use with adsorption vessels. The assembly consists of two valve members which have respectively engaged surfaces that are relatively rotatable to provide valving action.
The valve assembly of this patent permits fluid to flow to and from the various adsorption vessels at appropriate times during the process cycle. The disclosure of this patent is incorporated herein by reference.
Useful pressure swing adsorption plant designs which incorporate rotary valves are described in U.S. Pat. Nos. 5,268,021, 5,366,541 and RE 35099, the disclosures of all of which are incorporated herein by reference. Each of these patents disclose controlling the operation of a battery of two or more adsorption vessels during a PSA process by a rotary valve which directs feed to and desorbed component from the various adsorption vessels of the system. The rotary valve described in these patents also provides for the transfer of fluid from one vessel to another during a pressure equalization step. Pressure equalization is the passage of gas from a first vessel that has just completed its adsorption step to a vented or evacuated vessel which has just completed its adsorbent regeneration step. In the above patents the flow of fluid during pressure equalization is from the higher pressure vessel via its inlet end, then through the rotary valve, then into the low pressure vessel via its inlet end. This method of bed equalization, referred to herein as "inlet to inlet equalization", is not very efficient for certain adsorption processes because less of the fractionated gas near the outlet end of the first vessel is transferred to the second vessel. The gas remaining in the first vessel is lost during the following depressurization step.
More efficient operation of adsorption system operation is achieved when other pressure equalization techniques are employed. A particularly useful pressure equalization method is that known as "inlet-to-inlet/outlet-to-outlet equalization", during which fluid flows from the high pressure vessel to the low pressure vessel by parallel flow through both the inlets and the outlets of the vessels. This technique is described in detail in U.S. Pat. No. 5,176,722, the disclosure of which is incorporated herein by reference.
There is a need for rotary valve assemblies which can enable an adsorption system to operate with adsorption cycles which include the above-described inlet-to-inlet/outlet-to-outlet pressure equalization step. This invention provides rotary valve assemblies which have these capabilities, and which enable additional steps, such as product gas flow, vessel purging and product fluid backfill to be automatically controlled without additional valves.